This project deals with two aspects of site-specific recombination in Staphylococcus aureus. One system under investigation is Tn554, a transposable element that mediates resistance to erythromycin and spectinomycin. Tn554 is the prototype of a class of clinically significant unusual genetic elements in Staphylococcus that appear to be specifically mobilized by bacteriophages. At present our emphasis is directed toward an understanding of the unusual requirements and regulation of the transposition process of Tn554. Also under investigation is an invertible segment located on the staphylococcal penicillinase plasmid pI524. Invertible sequences have been demonstrated to be involved in the regulation of several bacterial and viral processes; in the case of pI524, the invertible segment is located adjacent to the genes for penicillinase production and is part of a putative transposable element. Current studies are directed toward an understanding of the function of the inversion and its relationship to penicillinase synthesis, and of the functions and sites involved in the recombination events.